In a typical cellular radio system, wireless terminals (also referred to as user equipment unit nodes, UEs, and/or mobile stations) communicate via a radio access network (RAN) with one or more core networks. The RAN covers a geographical area which is divided into cell areas, with each cell area being served by a radio base station (also referred to as a RAN node, a “NodeB”, and/or enhanced NodeB “eNodeB”). A cell area is a geographical area where radio coverage is provided by the base station equipment at a base station site. The base stations communicate through radio communication channels with UEs within range of the base stations.
Multi-antenna techniques can significantly increase capacity, data rates, and/or reliability of a wireless communication system as discussed, for example, by Telatar in “Capacity Of Multi-Antenna Gaussian Channels” (European Transactions On Telecommunications, Vol. 10, pp. 585-595, November 1999). Performance may be improved if both the transmitter and the receiver are equipped with multiple antennas to provide a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO. The LTE standard is currently evolving with enhanced MIMO support and MIMO antenna deployments. A spatial multiplexing mode is provided for relatively high data rates in more favorable channel conditions, and a transmit diversity mode is provided for relatively high reliability (at lower data rates) in less favorable channel conditions.
In a downlink from a base station transmitting from an antenna array over a MIMO channel to a wireless terminal, for example, spatial multiplexing (or SM) may allow the simultaneous transmission of multiple symbol streams over the same frequency from different antennas of the base station antenna array. Stated in other words, multiple symbol streams may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same downlink time/frequency resource element (TFRE) to provide an increased data rate. In a downlink from the same base station transmitting from the same antenna array to the same wireless terminal, transmit diversity (e.g., using space-time codes) may allow the simultaneous transmission of the same symbol stream over the same frequency from different antennas of the base station antenna array. Stated in other words, the same symbol stream may be transmitted from different antennas of the base station antenna array to the wireless terminal over the same time/frequency resource element (TFRE) to provide increased reliability of reception at the wireless terminal due to transmit diversity gain.
The performance of a wireless communication system can thus be improved using multiple antennas at the base station and/or wireless terminal to provide spatial multiplexing SM in more favorable channel conditions and to provide transmit diversity gain in less favorable channel conditions. Transmit diversity and/or spatial multiplexing may be implemented without knowledge of the wireless channel at the transmitter. In many wireless communication standards such as the 3rd Generation Partnership Project (3GPP), Long Term Evolution (LTE), High-Speed Downlink Packet Access (HSDPA), and/or Worldwide Interoperability for Microwave Access (WiMAX), however, knowledge of the wireless channel (referred to as channel state information or CSI) may be provided at the MIMO transmitter via feedback from the receiver as discussed, for example, in the 3rd Generation Partnership Project document entitled “UTRA-UTRAN Long Term Evolution (LTE) And 3GPP System Architecture Evolution (SAE)” (http://www.3gpp.org/Highlights/LTE/LTE.htm). Accordingly, the MIMO transmitter can use the channel state information (or CSI) to provide precoding to further improve system performance as discussed, for example, by Scaglione et al. in “Optimal Designs For Space-Time Linear Precoders And Decoders” (IEEE Transactions On Signal Processing, Vol. 50, No. 5, May 2002, pages 1051 to 1064) and by Sampath et al. in “Generalized Linear Precoder And Decoder Design For MIMO Channels Using The Weighted MMSE Criterion” (IEEE Transactions On Communications, Vol. 49, No. 12, December 2001, pages 2198 to 2206). CSI precoding can thus be used by a MIMO transmitter to provide beam forming gain and/or to condition transmissions to existing characteristics of the wireless channel.
Communications standards such as 3GPP, LTE, HSDPA, and/or WiMAX may use frequency division duplex (FDD) such that uplink and downlink communications between two communications devices (e.g., between a wireless terminal and a base station) are provided over different carrier frequencies. In such FDD MIMO systems (referred to as limited feedback systems), channel state information may be provided at a MIMO transmitter as feedback information from the receiving device with which it is communicating. For example, a MIMO transmitter at a base station may transmit over a downlink (using a first carrier frequency) to a wireless terminal, and the wireless terminal may provide channel state information for the downlink as feedback that is transmitted over the uplink (using a second carrier frequency different than the first carrier frequency) to the base station. In such limited feedback systems, CSI feedback may be provided, for example, using codebook based feedback and/or quantized channel feedback as discussed, for example, Mukkavilli et al. in “Design Of Multiple Antenna Coding Schemes With Channel Feedback” (Proc Annual Asilomar Conf. On Signal Systems And Components, 2001, pages 1009-1013) and by Love et al. in “Quantized Antenna Weighting Codebook Design For Multiple-Input Multiple-Output Wireless Systems” (Proc. 40th Allerton Conf. Communications, Control, And Computing, Moticello, Ill., 2002). Communications standards such as 3GPP, LTE, HSDPA, and WiMAX may use codebook based CSI feedback for precoding.
With codebook based precoding, a same codebook is defined at both the transmitting and receiving devices (e.g., at both the base station and wireless terminal). The codewords (also referred to as entries which may be matrices, vectors, etc.) may be constructed using methods such as Grasmanian, Lyod algorithm, Discrete Fourier transform DFT matrix, etc. At the receiving device (e.g., at the wireless terminal) Signal-to-Interference-and-Noise-Ratios (SINRs) may be calculated for received downlink signals for each of the codewords of the codebook, and a rank and a codeword providing the best performance (e.g., highest spectral efficiency and/or capacity) may be selected for CSI feedback. Because the codewords of the codebook are known at both the transmitting and receiving devices, the receiving device may include a precoding index in the channel state information that is provided to the transmitting device wherein the precoding index uniquely identifies the selected codeword and rank. By providing an index as opposed to the codeword, communication resources may be used more efficiently.
In general, the wireless terminal may decide whether to receive over a downlink in a MIMO diversity mode or in a MIMO multiplexed mode, and the wireless terminal reports this decision using a Rank Indicator (RI). For example, a Rank Indicator of 1 may specify a MIMO diversity mode providing only one symbol stream (also referred to as a transmission layer), a Rank Indicator of 2 may specify a MIMO multiplexed mode simultaneously providing two symbol streams (or transmission layers), a Rank Indicator of 3 may specify a MIMO diversity mode simultaneously providing three symbol streams (or transmission layers), a Rank Indicator of 4 may specify a MIMO multiplexed mode simultaneously providing four symbol streams (or transmission layers), etc. For example, with a four antenna base station transmitter, Rank Indicators from 1 to 4 may be available, and with an eight antenna base station transmitter, Rank Indicators from 1 to 8 may be available. Moreover, the codebook may include multiple codewords for each of the ranks With a four antenna base station transmitter, up to four different transmission layers defined by four respective Rank Indicators (e.g., Rank Indicators 1 to 4) may be available, and 16 codewords may be provided for each transmission layer or rank for a codebook of 64 codewords. With an eight antenna base station transmitter, up to eight different transmission layers defined by eight respective Rank Indicators (e.g., Rank Indicators 1 to 8) may be available, and 16 codewords may be provided for each transmission layer or rank for a codebook of 128 codewords.
Because MIMO downlink channel characteristics vary as a function of many factors including location of the wireless terminal relative to the base station, direction/speed of movement of the wireless terminal, natural and manmade terrain, etc., and because MIMO downlink characteristics and demand for downlink channel capacity may change during a communication, Rank Indicator (RI) and codeword calculations and channel information reporting may be performed by the wireless terminal over the entire band (for example, in 3GPP-LTE, Wi-Max, etc.) at given periods as configured by the base station. Processing overhead for channel state information may thus increase dramatically with increasing transmission layers/antennas.
Rank Indicator (RI or i) and codeword choice is generally a function of channel characteristics, correlation between transmit and receive antennas, etc. A conventional approach to choosing a Rank Indicator (RI or i) and codeword is to find SINRs for different transmission configurations, and to choose a Rank Indicator and codeword for a configuration providing a best performance. For example, a wireless terminal may compute an SINR for each possible value of RI/codeword and find spectral efficiencies using an exhaustive search of all possible configurations (i.e., using all possible codewords). The wireless terminal may then choose the Rank Indicator and codeword for the configuration providing the best performance (e.g., highest efficiency and/or capacity). The wireless terminal may then report the selected Rank Indicator (RI or i) and codeword to the base station in a Channel State Information (CSI) report. These operations may be particularly burdensome when the codebook is large.
While performance of a closed loop MIMO system may generally improve with larger codebooks, the requirement for the receiving device (e.g., wireless terminal) to periodically determine performances (e.g., efficiencies and/or capacities) for all possible downlink configurations (e.g., to periodically calculate performances for each possible codeword) may place a burden on wireless terminal processing capacity and/or may reduce wireless terminal battery life.